1. Field of the Invention
The present invention relates to a technology for feeding a recording medium in an image forming apparatus.
2. Description of the Related Art
When a recording sheet is picked up from a sheet tray and conveyed, multi feed may occur, necessitating the sheets to be separated one by one after being picked up. FIG. 8 is a schematic diagram of a feed roller arranging portion of a conventional sheet feeding device showing a relation between a feed roller 48 and a friction pad 49. The feed roller 48 is provided at the most downstream end of the sheet tray in a sheet feeding direction, and the friction pad 49 for separating recording sheets is provided on the downstream side in a sheet drawing direction. The friction pad 49 is made of a material that produces a kinetic frictional resistance that is at least greater than the kinetic frictional resistance between recording sheets. The friction pad 49 includes a spring 49a and a pad support 49d, and produces a designated kinetic frictional resistance by launching the pad support 49d, with the spring 49a imparting a predetermined contact pressure. Thus, when two recording sheets are conveyed at a time, a first recording sheet in contact with the feed roller 48 is separated from a second recording sheet in contact with the friction pad 49 because the kinetic frictional resistance between the recording sheets is smaller than that between the second recording sheet and the friction pad 49, and is conveyed towards a pair of registration rollers 46a and 46b along a sheet conveying path 42.
In the sheet feeding device having the structure described above, the recording sheet is only nipped at the point where the feed roller 48 and the friction pad 49 are in contact. In such a structure, stick-slip vibrations generated by the friction between the friction pad 49 and the recording sheet cannot be suppressed, so that conveying behavior and conveying accuracy do not become stable and abnormal sound is generated.
Various technologies have been known to prevent or suppress abnormal sound at the time of separating recording sheets, such as those disclosed in Japanese Patent No. 3911113, Japanese Patent Application Laid-open No. 2006-306597, and Japanese Patent Application Laid-open No. 2002-154694.
Japanese Patent No. 3911113 discloses a sheet separating/feeding device, that separates the leading edges of a plurality of sheets that are conveyed from upstream of a friction pad in a sheet conveying direction in an overlapping manner, separates the sheets one by one at a nip between a rotating feed roller and the friction pad downstream of the friction pad in the sheet conveying direction, and feeds the separated sheets. The friction pad is mounted on a pad holder in such a way that the surface area of the friction pad is less on the downstream side than on the upstream side in the sheet feeding direction. Furthermore, concaves and convexes are formed on the surface of the pad holder on the downstream side of the sheet feeding direction to set a coefficient of friction to the pad holder on the downstream side.
Japanese Patent Application Laid-open No. 2006-306597 discloses a sheet separating device that separates sheets fed from a sheet stack contained in a sheet cassette one by one. The sheet separating device includes a separation roller that picks up the topmost sheet by coming into pressure-contact with the sheet and a slanting member that includes a slant portion that is in pressure-contact with the separation roller and applies resistance to the leading edge of the sheet with respect to a direction of a sheet entry route.
Japanese Patent Application Laid-open No. 2002-154694 discloses a sheet feeding device that includes a pickup roller that is arranged in contact with the lower side of a sheet, and a swingable pressure plate that comes into contact with the upper side of the sheet via a sheet member and presses the sheet against the pickup roller. The sheet is fed in a predetermined direction while being nipped by a nip formed by the pickup roller and the pressure plate via the sheet member. An intermediate sheet member is provided between the pressure plate and the sheet member.
Although the technologies described above intend to prevent multi feed and abnormal sound, it is hard to sufficiently suppress or prevent abnormal sound due to the stick-slip vibrations.